ML13255A421
| ML13255A421 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 09/20/2013 |
| From: | Travis Tate Plant Licensing Branch III |
| To: | Lieb R FirstEnergy Nuclear Operating Co |
| Brown E 301-415-2315 | |
| References | |
| L-13-076, TAC MF0752 | |
| Download: ML13255A421 (9) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555*0001 September 20, 2013 Mr. Ray Lieb Site Vice President FirstEnergy Nuclear Operating Company Mail Stop A-DB-3080 5501 North State, Route 2 Oak Harbor, OH 43449-9760
SUBJECT:
DAVIS-BESSE NUCLEAR POWER STATION, UNIT NO.1 - SAFETY EVALUATION FOR RELIEF REQUEST RR-A37 (TAC NO. MF0752) (L-13-076)
Dear Mr. Lieb:
By letter dated February 27, 2013, as supplemented by letter dated July 24, 2013 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML13059A321, and ML13206A308, respectively), FirstEnergy Nuclear Operating Company (FENOC), the licensee, submitted request RR-A37 to the Nuclear Regulatory Commission (NRC). The licensee requested relief from the requirements of the American Society of Mechanical Engineers (ASME)
Boiler and Pressure Vessel Code (Code) Case N-770-1, as conditioned by section 55a(a)(g)(6)(ii)(F) of Title 10 of the Code of Federal Regulations Part 50 (10 CFR 50), at Davis-:.Jesse Nuclear Power Station (Davis-Besse), Unit 1. Specifically, in lieu of the essentially 100 percent ultrasonic (UT) examination coverage requirement for reactor coolant system cold-leg drain line 1-2 dissimilar metal weld (DMW) overlay, the licensee proposed crediting the aggregate examination coverage achieved in the spring 2012 UT examination.
The NRC staff determined that the proposed alternative provides reasonable assurance of structural integrity and leak tightness, and that complying with the specified requirement would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety Accordingly, the NRC staff concluded that the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a( a)(3)(ii) and authorizes the use of RR-A37, crediting the spring 2012 UT examination of cold-leg drain line 1-2 DMW and full structural weld overlay to fulfill the ASME Code Case N-770-1 requirement for the UT examination that is to be performed in the first or second refueling outage following overlay at Davis-Besse, Unit 1.
This relief is authorized for the third 10-year inservice inspection interval at Davis-Besse, Unit 1, which began on September 20,2000, and ends on September 20,2013, based on a 2-year extension to the interval due to the 2-year shutdown foliowing the unit's 13th operating cycle and an ad~~llonal i-year extension consistent with ASME Code,Section XI, IWA-2430(d).
All other ASME Code,Section XI, requirements for which relief was not specifically requested and authorized in the subject proposed alternative remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
R. Lieb
- 2 Should you have any questions, please feel free to contact Ms. Eva Brown at (301) 415-2315.
Sincerely, Travis L. Tate, Chief Plant Licensing Branch 111-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-346
Enclosure:
Safety Evaluation cc w/encl: Distribution via Listserv
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELIEF REQUEST NO. RR-A37, REACTOR COOLANT SYSTEM COLD-LEG DRAIN LINE DISSIMILAR METAL WELD EXAMINATION FIRSTENERGY NUCLEAR OPERATING COMPANY DAVIS-BESSE NUCLEAR POWER STATION, UNIT 1 DOCKET NO 50-346 10 INTRODUCTION By letter dated February 27,2013, as supplemented by letter dated July 24,2013 (Agencywide Documents Access and Management System (ADAMS) Accession Nos. ML13059A315 and ML13206A308, respectively), FirstEnergy Nuclear Operating Company (the licensee) submitted request RR-A37 to the Nuclear Regulatory Commission (NRC) for review and authorization. The licensee requested relief from the requirements of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code) Case N-770-1, as conditioned by Section 50.55a(a)(g)(6)(ii)(F) of Title 10 of the Code of Federal Regulations (10 CFR), at Davis-Besse Nuclear Power Station (Davis-Besse), Unit 1. Specifically, in lieu of the essentially 100 percent ultrasonic (UT) examination coverage requirement for reactor coolant system cold-leg drain line 1-2 dissimilar metal weld (DMW) overlay, the licensee proposes crediting the aggregate examination coverage achieved in the spring 2012 UT examination. The licensee requested authorization to use the proposed alternative pursuant to 10 CFR 50.55a(a)(3)(ii) on the basis that complying with the specified requirement would result in hardship or unusual difficulty without a cornpensating increase in the level of quality and safety.
2.0 REGULATORY EVALUATION
Section 50.55a(g), Inservice Inspection Requirements, to 10 CFR, states that ASME Code Class 1, 2, and 3 components (including supports) must meet the requirements, except the design and access provisions and the pre-service examination requirements, set forth in the ASME Code,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, to the extent practical within the limitations of design, geometry, and materials of construction of the components. The regulations require that inservice examination of components and system pressure tests conducted during the first 1 O-year inspection interval and subsequent 10-year inspection intervals comply with the requirements in the latest edition and addenda of Section XI of the ASME Code incorporated by reference in 10 CFR 50.55a(b) 12 months prior to the start of the 120-month inspection interval, subject to the limitations and modifications listed therein.
Enclosure
- 2 The requirements in 10 CFR 50. 55a(g)(6)(ii)(F)( 1) state that existing pressurized-water reactors (PWRs) in operation:
... as of July 21, 2011 shall implement the requirements of ASIVlE Code Case N-770-1, subject to the conditions specified in paragraphs (g)(6)(ii)(F)(2) through (g)(6)(ii)(F)(10) of this section, by the first refueling outage after August 22, 2011.
Section 50.55a(a)(3) of 10 CFR states, in part, that alternatives to the requirements of 10 CFR 50.55a(g) may be used, when authorized by the NRC, if (i) the proposed alternatives would provide an acceptable level of quality and safety or (ii) compliance with the specified require~ents would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
3.0 TECHNICAL EVALUATION
3.1 Licensee's Request for Alternative Components for Which Relief is Being Requested Reactor coolant system cold-leg drain line 1-2 DMW overlay, weld number R-40-CCA-18-3-FW9, ASME Code Class 1, ASME Code Case N-770-1 Inspection Item "F."
Code Requirements The code of record for the third 1 O-year inservice inspection (lSI) interval is the 1995 Edition through the 1996 Addenda of the ASME Code,Section XI. As a result of a 2-year extension, due to an unscheduled 2-year shutdown following the unit's 13th operating cycle, and an additional 1-year extension as permitted by IWA-2430(d) of Section XI of the ASME Code, the third 10-year lSI interval, which started on September 21, 2000, is scheduled to end on September 20, 2013.
ASME Code Case N-770-1, "Alternative Examination Requirements and Acceptance Standards for Class 1 PWR Piping and Vessel Nozzle Butt Welds Fabricated with UNS N06082 or UNS W86182 Weld Filler Material with or without Application of Listed Mitigation Activities,"
paragraph -2500(a) requires, in part, that "[w]elds shall be examined as specified in Table 1." Per Table 1, "Examination Categories," Inspection Item F, UT examination is required once during the first or second refueling outage following overlay. Note (8)(a) to Table 1 states:
The weld overlay examination volume in Fig. 2(a) shall be ultrasonically examined to determine the acceptability of the weld overlay and to determine if any new or existing cracks have propagated into the outer 25% of the original weld or base material or into the overlay. The angle beam shall be directed perpendicular and parallel to the piping axis, with scanning performed in four directions.
Essentially 100 percent UT examination coverage is required per 10 CFR 50.55a(g)(6)(ii)(F)(4).
- 3 Licensee's Proposed Alternative and Basis In lieu of the essentially 100 percent UT examination coverage requirement, the licensee proposes crediting the aggregate coverage achieved from the spring 2012 examination, of approximately 70 percent. The licensee states that, due to the configuration/geometry of this nozzle-to-elbow weld, achieving essentially 100 percent coverage of the examination volume is unattainable without a significant modification to the existing piping. Implementing this type of modification would require a core offload, flushing and draining of the system, and removing/replacing the existing piping with reconfigured piping. Based on radiological survey results from the spring 2012 refueling outage, this work would occur within radiation fields with dose rates up to 500 mil Ii-roentgen equivalent man (mR) per hour, resulting in a significant increase in occupational radiation exposure [dose] to personnel.
3.2
NRC Staff Evaluation
Primary water stress-corrosion cracking (PWSCC) of nickel-based pressure boundary materials is a safety concern. Operational experience has shown that PWSCC can occur as the result of the combination of susceptible material, corrosive environment, and tensile stress, resulting in the potential for leakage and loss of structural integrity_ The subject DMW, which jOins carbon steel to stainless steel, is made of nickel-based Alloy 82/182 material and this alloy is susceptible to PWSCC in the PWR environment. A full structural weld overlay (FSWOL) of PWSCC-resistant Alloy 52M weld metal has been deposited on the outside surface of the Alloy 82/182 DMW to mitigate the PWSCC susceptibility and form a new pressure boundary that does not rely on the integrity of the original DMW. The examination requirements of ASME Code Case N-770-1, as conditioned by 10 CFR 50.55a(g)(6)(ii)(F), are intended to ensure the structural integrity and leak tightness of the DMW and the FSWOL through nondestructive examination.
The licensee states that. due to the configuration/geometry of this nozzle-to-elbow weld, achieving essentially 100 percent UT examination coverage of the weld overlay volume shown in Fig. 2(a) of ASIVIE Code Case N-770-1 is not achievable without significant modification of the existing piping. In the supplement dated July 24, 2013, the licensee submitted scale drawings of the nozzle-to-elbow DMW and FSWOL showing the limits of the UT transducer movement and the examination coverage obtained, and provided the UT examination procedure Performance Demonstration Qualification Summary (PDQS) No. 651.
The NRC staff examined the licensee's drawings and reviewed the PDQS. The NRC staff observed that the physical geometry of the weld/FSWOL and the 45-degree minimum UT scan angle noted in PDQS No. 651 for lSI examination of component diameters less than 4 inches (pre-overlay) does not permit essentially 100 percent LIT examination coverage. The licensee further states that modification of the subject piping to obtain essentially 100 percent UT examination coverage would require a core offload, flushing and draining of the system, and removing/replacing the existing piping with reconfigured piping.
The licensee indicated that based on radiological survey results from the spring 2012 refueling outage that this work would result in a significant increase in occupational radiation exposure
[dose] to personnel. The NRC staff questioned the low dose (32 mR) recorded on the Examination Data sheet for the May 15, 2012, examination radiological given the dose rate cited.
- 4 The licensee indicated in the supplement dated July 24, 2013, that the 500 mRlhour dose rate is measured on contact with the cold-leg drain pipe. As a result, any modifications to the pipe and associated preparation activities would occur with radiation fields with dose rates up to 500 mRlhr. Given the high dose rates for modification and support activities, the NRC staff finds that the radiological dose associated with modification of the existing piping to achieve essentially 100 percent UT examination coverage would constitute a hardship.
The DMW joins a carbon steel nozzle (A-105, Grade 2) to a stainless steel elbow (SA-403, Grade WP316) using Alloy 82/182 weld metal. In the supplement dated July 24,2013, the licensee provided UT scan coverage maps detailing the UT scan coverage attained and the UT examination volume required by ASME Code Case N-770-1. The licensee also submitted tables detailing the UT examination coverage of the carbon steel nozzle material, susceptible weld metal, stainless steel base material, and the FSWOL material at four azimuthal locations, 0, 90, 180, and 270 degrees, for scans in the axial and circumferential directions. The licensee's circumferential scan coverage calculations credited 100 percent examination coverage for the volume interrogated in two directions. The licensee's axial scan coverage calculations credited 100 percent examination coverage for the volume interrogated by axial scans in two directions, but only 50 percent coverage for the volume examined by the axial scans in only one direction.
When the UT scan coverage was calculated in this manner, the licensee calculated a composite UT scan coverage for the four azimuthal locations of 71 percent of the required volume for circumferential scans, and 69 percent for axial scans, significantly less that essentially 100 percent required by ASIVIE Code Case N-770-1. The licensee also provided sketches of the positions of the UT probes at the extreme positions of the travel on the FSWOL. The NRC staff has examined the scan coverage maps and the drawings and finds that the scan coverage values provided by the licensee are reasonable, thus accepts the values provided by the licensee.
I Azimuthal.
Axial UT Scans Circumferential UT Scans i Material Position 0
0 90 0
180 0
270 0
Average 0° 90° 1§27O O
Average I Carbon Steel Nozzle 100 100 72 66 84.5 100 100 1
100 100
- DMW 65 62 75 65 66.75 92 91 78 100 90.25 Stainless Steel Elbow 50 50 40 50 47.5 0
010 0
0
,FSWOL 73 71 71 75 72.5 74 70 72 73 72.25 The NRC staff has reviewed the individual volumes of materials interrogated and evaluated the potential consequences of missed UT examination scan coverage. The carbon steel base metal and stainless steel base metal were examined in the spring 2010 refueling outage and the examination found no suspected flaws (see April 29, 2010, commitment closeout at ADAMS Accession No. ML101230641). Based on the examination conducted in 2010 and the fact that these materials are not known to be susceptible to PWSCC or other service-related cracking in the PWR coolant environment, the NRC staff finds the use of the reasonably achievable scan coverage acceptable. The spring 2010 UT examination also examined the FSWOL and found no suspected flaws. Since the FSWOL was examined in spring 2010 and is not in contact with primary water, there is no credible service-related degradation mechanism.
- 5 Therefore, the NRC staff finds that the lack of full ASME Code,Section XI, Appendix VIII-compliant UT examination coverage of the FSWOL is not a structural integrity concern.
An inside diameter surface-connected flaw in a PWSCC-susceptible DMW is a structural integrity concern. The FSWOL was employed to mitigate PWSCC cracking that exists in the underlying DMW and was designed to form a new pressure boundary that does not rely on the DMW to carry the anticipated loads. In addition to providing structural reinforcement to the PWSCC-susceptible DMW, the FSWOL produced compressive residual stresses that mitigate PWSCC in the underlying DMW and employed PWSCC-resistant material.
In a letter dated January 21,2010 (ADAMS Accession No. ML100080573), the NRC staff authorized application of the FSWOL for Davis-Besse, Unit 1. In support of this review, the licensee committed to perform a crack growth analysis to determine the time period for cracks to grow to the design basis. On June 18, 2010, the licensee provided a report summarizing the design and analyses for various welds (Summary Report) (ADAMS Accession No. ML101740042), which states that the design basis flaw assumption for the FSWOL is a circumferentially oriented flaw that extends through the original component wall thickness for 360 degrees. Once the FSWOL is applied, the design-basis flaw becomes the allowable flaw size for the end-of-evaluation period, and any actual observed or postulated flaws in the DMW must be demonstrated by a crack growth calculation not to grow beyond the allowable size before the next scheduled lSI.
For the case of the FSWOL, the UT examination volume includes the weld overlay plus the outer 25 percent of the DMW original wall thickness for both axial and circumferential flaws. Therefore, an inside diameter surface-connected flaw that is 75 percent of the original weld thickness is assumed as the largest flaw that could escape detection by the examination, and the length of time required for the assumed initial flaw to grow to the DMW-FSWOL interface is the design life.
In the Summary Report, the licensee states that the total sustained stress intensity factor during normal plant operation was determined for the assumed crack depth considering internal pressure stresses, residual stresses, steady state thermal stresses, and stresses due to sustained piping loads (including deadweight). The Summary Report also indicates that the time for the postulated 75 percent through-wall flaw to propagate to the DMW-FSWOL interface by PWSCC is greater than 60 years for circumferential-oriented cracks and approximately 14 years for axially oriented cracks. The NRC staff performed independent confirmatory analysis of PWSCC flaw growth in response to weld residual stresses, including those that result from the application of the FSWOL, and internal pressure stresses. The initial flaw depth was assumed to be 75 percent of the initial DMW thickness. The results of the confirmatory analysis support the times in the licensee's Summary Report.
The NRC staff notes that the basis for the flaw growth calculation, ASME Code-compliant UT examination coverage of the outer 25 percent of the original DMW, has not been achieved.
Circumferential UT scans achieved essentially 100 percent coverage, as defined in ASME Code Case N-460, "Alternative Examination Coverage for Class 1 and Class 2 Welds," of the required volume of the susceptible DMW material, but the axial UT scans could not achieve the required examination coverage.
-6 The licensee's procedure PDQS No. 651 indicates that the UT procedure is restricted to scan angles greater than or equal to (~) 45 degrees for scanning component diameters less than 4 inches (pre-overlay). However, the NRC staff notes that the UT examination employed a phased array probe with UT scan angles between 0 and 80 degrees generated at 1 degree increments. Review of the licensee vendor's Phased Array Calibration Data Sheet shows that each angle range (i.e., 70 to 85 degrees, 25 to 60 degrees, and 0 to 25 degrees). was calibrated with different gains and the Phased Array Weld Overlay Examination Data Sheet shows "no recordable indications" for axial scans performed from 0 to 80 degrees and circumferential scans performed from 0 to 85 degrees. Although the licensee cannot claim credit for UT examination using angles < 45 degrees, the NRC staff notes that axial scans at shallow angles interrogate a significantly larger volume than those for angles ~ 45 degrees. The NRC staff further notes that the required volume of the DMW was scanned in at least one axial scan direction, and essentially 100 percent of the required UT examination volume was scanned in both circumferential scan directions. The NRC staff finds that there is reasonable assurance that if a crack had existed in the outer 25 percent of the susceptible DMW material, it would have been detected. The NRC staff concludes that there is reasonable assurance of structural integrity of the cold-leg drain line 1-2 DMW and FSWOL.
As a result of these examination coverage and degradation considerations, the NRC staff finds that there is reasonable assurance of the structural integrity and leak tightness of the DMW and FSWOL. The NRC staff also finds that requiring the licensee to perform the piping modifications necessary to permit an ASME Code-compliant examination would result in a hardship without a compensating increase in the level of quality and safety.
4.0 CONCLUSION
Based on the above, the NRC staff concludes that the proposed alternative, RR-A37, provides reasonable assurance of structural integrity and leak tightness, and that complying with the specified requirement would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a(a)(3)(ii) and, therefore, authorizes crediting the spring 2012 UT examination of cold-leg drain line 1-2 DMW and FSWOL to fulfill the ASME Code Case N-770-1 requirement for the UT examination that is to be performed in the first or second refueling outage following overlay at Davis-Besse, Unit 1.
All other ASME Code,Section XI, requirements for which relief was not specifically requested and authorized in the subject proposed alternative remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributor: Jay Wallace, NRRlDE/ESGB Dated: September 20, 2013
ML13255A421 NRR-028 OFFICE LPL3-2/PM LPL3-2/LA EPNB/BC LPL3-2/BC NAME EBrown SRohrer (SFigueora for) ~Old TTate DATE 9/20/13 9/13/13 113 9/20/13